Conventionally, establishing communication between terminals in an integrated services digital network (ISDN) for the purposes of videophone calls, involves using high data-rate lines between interconnected terminals for the purposes of transmitting video signals, because of the quantity of information contained in each image.
Although it is technically possible to make such lines available to users, it is not currently economically viable, since the cost is prohibitive.
This has led to research into reducing the quantity of information to be transmitted, in particular by means of image data compression and time prediction techniques, so that only information that is essential and that is in the most compact form possible is transmitted.
The article "Visages" by J. Guichard and G. Eudes published in 1990 in issue No. 140 of the review "L'Echo des Recherches", published in Issy-les-Moulineaux, France, mentions a research program aimed at enabling animated images to be transmitted between videophones via channels whose data rate is equal to or a multiple of 64 Kbit/s, in compliance with CCITT Recommendation H261. That research considers individually connecting each of the videophones to an ISDN network basic access which offers a useful data rate of 144 Kbit/s divided between two 64 Kbit/s B channels and one 16 Kbit/s D channel.
Although in that way a considerable reduction can be made in the digital data rate required for transmitting animated images between videophones temporarily interconnected by means of an ISDN network, each of the videophones must nevertheless be capable both of compressing the image information that it picks up locally, prior to transmitting the information in compressed form, and of decompressing the image information that is reconstituted locally after it has been transmitted over the ISDN network.
As a result, the complexity of the operations to be performed on the image information bits and the volumes of memory required for storing said bits temporarily are such that the compression operation and the decompression operations are conventionally performed by means of units that are physically distinct, referred to respectively as "video encoding units" and "video decoding units" which are complex and involve a large number of components.
Those components are not organized optimally, and prior art codecs are therefore costly and bulky, and as a result they are not viable for applications, such as videophone systems, intended for mass consumption.